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The authors determined the level of zonal and dendritic segregation in slabs poured by thin-slab technology. The calculated variation coefficients of the content of basic and impurity chemical elements over the slab cross-section do not exceed 10 %, zonal segregation is low. The manganese content measured with the area occupied by the dendritic axes and the interstitial spaces showed the level of dendritic segregation. The manganese concentration varies from 0.6 to 1.1 %, respectively. It was established that the use of dynamic soft compression during solidification makes it possible to grind the primary dendritic structure to form additional centers during the phase transformation of δ-ferrite into austenite. Dimensions of the initial austenitic grains formed taking into account the primary dendritic structure are 3 times smaller in a thin slab than in a slab with a thickness of more than 200 mm. Transformations of the dendritic structure during compression show high workability necessary for the formation of uniform austenitic grains in the fullering before finishing rolling. The study has not confirmed the hypothesis that bainite of coarse morphology in the microstructure of hot-rolled products is formed in segregation areas. The hereditary influence of the primary dendritic structure on the structure formation during rolling was revealed. The manganese concentration varies between the bainite and the “neighboring” structure from 0.68 to 1.01 %, similar to the level of the initial dendritic segregation. Difference in the content of chemical elements affects the processes of recrystallization of austenitic grains during high-temperature rough rolling. Bainite was formed within the framework of chemically “depleted” large austenitic grains that are stable during phase transformation.
The authors determined the level of zonal and dendritic segregation in slabs poured by thin-slab technology. The calculated variation coefficients of the content of basic and impurity chemical elements over the slab cross-section do not exceed 10 %, zonal segregation is low. The manganese content measured with the area occupied by the dendritic axes and the interstitial spaces showed the level of dendritic segregation. The manganese concentration varies from 0.6 to 1.1 %, respectively. It was established that the use of dynamic soft compression during solidification makes it possible to grind the primary dendritic structure to form additional centers during the phase transformation of δ-ferrite into austenite. Dimensions of the initial austenitic grains formed taking into account the primary dendritic structure are 3 times smaller in a thin slab than in a slab with a thickness of more than 200 mm. Transformations of the dendritic structure during compression show high workability necessary for the formation of uniform austenitic grains in the fullering before finishing rolling. The study has not confirmed the hypothesis that bainite of coarse morphology in the microstructure of hot-rolled products is formed in segregation areas. The hereditary influence of the primary dendritic structure on the structure formation during rolling was revealed. The manganese concentration varies between the bainite and the “neighboring” structure from 0.68 to 1.01 %, similar to the level of the initial dendritic segregation. Difference in the content of chemical elements affects the processes of recrystallization of austenitic grains during high-temperature rough rolling. Bainite was formed within the framework of chemically “depleted” large austenitic grains that are stable during phase transformation.
The main direction in solving the problem of increasing the reliability of field equipment, is the creation of new steels with higher resistance to corrosion-mechanical destruction. Currently, to produce oil and gas pipeline systems, low-carbon, low-alloy steels are used, in which lath carbide-free bainite is formed when quenched in water. Such a structure provides a combination of high strength and resistance to brittle fracture. However, issues of increasing corrosion resistance are still open. The purpose of this work is to identify the structural condition of low-carbon, low-alloy, pipe steels, providing a combination of high mechanical properties with increased corrosion resistance in oilfield environments. The studies were carried out on the latest generation 08KhFA, 08KhFMA and 05KhGB steels, most popular when manufacturing oil and gas pipelines. Samples for the study were cut from the pipes and quenched from the austenite region in water, which formed the structure of lath carbide-free bainite. The quenched samples were tempered at temperatures of 200, 300, 400, 500, 600, and 700°C. To identify the relationship between the morphology of bainite structures and their properties, the samples after quenching and tempering at each temperature, were subjected to metallographic analysis, X-ray diffraction analysis, mechanical tests, and corrosion resistance tests. The work shows the sequence of structure transformation, temperature ranges of phase and structural transformations, changes in mechanical properties, and corrosion resistance that occur during tempering of lath carbide-free low-carbon bainite. It is shown that tempering of lath carbide-free bainite (08KhFA, 08KhMFA and 05KhGB steels) does not affect the rate of carbon dioxide corrosion. It has been found that medium tempering forms the structural condition of carbide-free low-carbon lath bainite providing a combination of high mechanical properties and high corrosion resistance in oil field environments. For each of the steels under study, the authors give recommended heat treatment modes.
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